A) Field of the Invention
This invention relates to a solid state imaging module and more specifically relates to a CCD area sensor for a digital still camera and a cellular phone.
B) Description of the Related Art
Conventionally, a CCD area sensor for a digital still camera and a cellular phone generally uses a substrate drain type electronic shutter for setting an exposure time.
That type of the CCD area sensor has photodiodes (photo electronic conversion elements) and transfer channels made of n-type regions formed in a p-well formed in an n-type substrate NSUB. The maximum (saturation) amount of electric charges stored in the photodiode is defined by an electrical potential of the substrate NSUB (n-type substrate). That is, if the electrical potential of the substrate NSUB is low, then the saturation electric charge amount is large, and when the electrical potential of the substrate NSUB gets higher, the saturation electric charge amount becomes small. When the electrical potential of the substrate NSUB gets extremely higher, all the electric charges accumulated in the photodiode are drained to the substrate.
By using the above-described property, the electronic shutter for starting electric charge accumulation in the photodiode is performed by impressing a high voltage to the substrate NSUB at the initial state of the exposure to drain the electric charges accumulated in the photodiode, and then returning the electrical potential of the substrate NSUB to the original voltage.
The electrical potential of the substrate NSUB is defined by the saturation electric charge amount of the photodiode during the exposure, and it is set to about 10V. Moreover, an impressing voltage for draining excessive electric charges to the substrate is generated by multiplying an amplitude pulse at about 20V to the voltage for the above-described operation.
FIG. 7 shows a circuit for supplying a NSUB voltage according to the prior art. As shown in the drawing, the circuit is consisted of bleeder resistors R1 and R2 and a diode D1 for generating DC bias, a condenser C1 for AC coupling and a clock driver 10.
During the exposure, a DC voltage (about 10V) defined by the resistors R1 and R2 is impressed to the NSUB. At this time, an output of the clock driver 10 is at a low level. The output of the clock driver 10 becomes a high level just before the next exposure is started after the electric charges accumulated in the photodiodes are read-out, the NSUB potential becomes high (for example, 32.5V) by the amplitude (for example, 22.5V) of the clock driver by C-coupling, and the excessive electric charges which have been accumulated just before that are drained to the NSUB. At this time, the diode D1 is turned off by reverse bias so that the bleeder resistors and the NUSB are electrically disconnected.
Although the DC voltage level impressed to the NUSB during the exposure defines the saturation output electric charge amount of the photodiode, the relationship between them varies according to unevenness of manufacturing processes; therefore, a recent CCD area sensor has built-in bleeder resistors and a fuse and is set to output a unique DC voltage by adjusting it at the time of shipping inspection.
The saturation level of the photodiode of CCD area sensor to date does not have to be changed according to image capturing modes because all pixel reading out has been a premises for a still picture and a movie. However, it becomes difficult to capture a movie with reading out all the pixels due to increase in the number of the pixels. Therefore, thinning out the number of pixels is performed by not reading out electric charges from a certain number of pixels to a transfer channel or mixing (the adding) outputs of two pixels in the transfer channel is performed at the time of taking a movie.
When the output electric charges of the pixels are added in the transfer channel, the electric charges may be overflow from the transfer channel if the saturation output level is high because the transfer channel has to deal with the electric charges of two pixels. Therefore, when the pixel output electric charges are added in the transfer channel, the saturation level of the photodiode has to be low.
Therefore, it has been suggested in the Japanese Laid-open Patent No. 2002-26186 that the NSUB voltage level is switched according to an imaging mode. When the pixel output electric charges are added in the transfer channel, a barrier formed by the p-well is lowered by increasing the NSUB electric potential to lower the saturation output level of the photodiode. When the pixel output electric charges are not added in the transfer channel (i.e., in an all pixel mode), the NSUB electric potential is lowered to increase the saturation output level of the photodiode.
FIG. 8 shows a circuit for switching the NSUB voltage levels according to the prior art. In the circuit shown in the drawing, an external resistor R3 is equipped, and connection and disconnection of the resistor R3 is switched by a mode signal “Mod” to switch between two levels of the DC voltage.
If the bleeder resistor is simply consisted of the resistors R1 and R2, the DC bias is defined only by a resistance ratio not depending on absolute value of the resistance of each resistor. However, if the external resistor R3 is added as in the above-described circuit for switching the NSUB voltage levels, the absolute value of each resistor is required to be precise for defining two voltage levels exactly, and therefore, a production output may be lowered.
Moreover, when all the resistors R1, R2 and R3 are not made of the same material, the DC bias may be changed due to differences in temperature coefficients of the resistors, and therefore to avoid that, all the resistors R1, R2 and R3 have to be built in the CCD module.
Further, it is possible to remove the circuit for generating the bias voltage from the CCD area sensor and to equip the circuit with a peripheral circuit. In this case, a manufacturing cost will increase because it becomes necessary to execute adjustment for absorb individual difference of the CCD area sensor after the CCD area sensor is assembled.
When a power source is turned on or turned off, the NSUB electrical potential may be negative. Normally. The CCD area sensor is formed in the p-well of the n-type substrate, and the p-well is grounded. When the n-type substrate becomes negative potential, a pn-junction is forward biased, and excessive current may cause damage in reliability or in an extreme case break down of the CCD area sensor.